1. Field of the Invention
The present invention relates to an electrical connecting device and a method for making the same and, more particularly relates to such an electrical connecting device, which can be advantageously used, such as, when a circuit board having a number of very fine electrical contacts in small spaces should be electrically contacted to another circuit board having corresponding electrical contacts. Also, the present invention relates to an electrical connecting device and a method for making the same, in which compact, high density and low contact resistance connection can be achieved without necessity of special force for inserting or removing the circuit boards to or from each other, when they are electrically connected.
2. Description of Related Art
A conventionally known electrical connecting device comprises, in general, a contact pin array referred to as "male contact" or "plug" which is inserted into or removed from spring means referred to as "female contact" or "jack". In this kind of electrical connecting device, when the plug is inserted into the jack, an electrical connection by metal-to-metal contact can be attained due to the mechanical friction between the both members. Although the force exerted by the spring depends on the material or the surface treatment layer of the spring means, the force is to be designed as at least ten and several grams to obtain low and stable contact resistance.
In recent years, in order to obtain a smaller sized and high performance electronic apparatus, such a mounting technique has been widely used that a plurality of elements, such as highly integrated LSIs, chip capacitors, or chip resistors, are mounted on a single circuit board. In these circuit boards, a number of input/output terminals for a single circuit board has been significantly increased. Such input/output terminals are, in general, contact pins (corresponding to plugs) which are fixed to the board by blazing or soldering. Therefore, when these plurality of contact pins are inserted into the jacks or withdrawn from the jacks, a large amount of force is necessary. At most, more than 100 kg is necessary and, therefore, the manually handling is sometimes difficult, the contact pins may be bent, or the blazed or soldered portion may be damaged.
To reduce the inserting and withdrawing force to overcome the above-mentioned problems, a connecting device using an auxiliary means, such as cam actuation mechanism or the like, with no or less inserting force has been recently developed and employed. However, the mount efficiency of the system may be reduced, since the construction of such a device is very complicated and a space for such cam actuation mechanism cannot be ignored.
To overcome these problems, U.S. Pat. No. 5,017,738 discloses an electrical connecting apparatus in which porous silver plating layers are provided on surface pads of a printed circuit board and on a surface of a contact provided on the pads of a mother board. These layers consist of first and second metals which forms an eutectic low-melting point metal when they are come into contact to each other. In Unexamined Patent Publication (kokai) No. 4-12588 discloses an electrical connecting apparatus using easily melting metals, such as indium and gallium, which are in solid state in the room temperature and in liquid state when used at a circuit actuating temperature.
As mentioned above, it has been already known that a metal or alloy which is in liquid state (liquid phase) in the room temperature or adjacent thereto is used as a material of contact. Also, it has been known that, as means for creating a liquid phase, one and the other metals for cooperating to constitute an alloy are formed separately on the contact members, and a liquid phase is created due to metal reaction or diffusion, when they are in contact with each other.
However, according to the above-mentioned solutions, it is relatively difficult to form a metal layer as a contact member on a board. Also, it is difficult to retain the metal for liquid phase on a predetermined position on a circuit board. Therefore, the above-mentioned prior art solutions have some problems in productivity and reliability.